A combined-cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined-cycle gas turbine (CCGT) plant, which is a kind of gas-fired power plant. The same principle is also used for marine propulsion, where it is called a combined gas and steam (COGAS) plant. Combining two or more thermodynamic cycles improves overall efficiency, which reduces fuel costs.
The principle is that after completing its cycle in the first (usually gas turbine) engine, the working fluid (the exhaust) is still hot enough that a second subsequent heat engine can extract energy from the exhaust. Usually the heat passes through a heat exchanger so that the two engines can use different working fluids.